Today, electronic devices such as smart cards are widely used in society. For example, smart cards may be used as electronic identity (eID) cards and payment cards bank cards). Although these cards are relatively secure due to their cryptographic capabilities, they are usually not equipped with a user interface suitable for entering user credentials, i.e. a so-called “authentication interface”. As a consequence, most card users still enter their PIN code through personal computers and laptops, which increases the risk that their credentials are intercepted by malicious software such as Trojans and key-logger programs. Some smart cards have been equipped with embedded touch-based user interfaces, such as capacitive sensor structures. However, it may be difficult to implement a touch-based interface that accurately captures touch events while using only a small amount of power.